Wafer bonded piezoresistive and piezoelectric force sensor and related methods of manufacture

ABSTRACT

Described herein is a ruggedized microelectromechanical (“MEMS”) force sensor. The sensor employs piezoresistive or piezoelectric sensing elements for force sensing where the force is converted to strain and converted to electrical signal. In one aspect, both the piezoresistive and the piezoelectric sensing elements are formed on one substrate and later bonded to another substrate on which the integrated circuitry is formed. In another aspect, the piezoelectric sensing element is formed on one substrate and later bonded to another substrate on which both the piezoresistive sensing element and the integrated circuitry are formed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application filed under 35 U.S.C. § 371 of PCT/US2018/044049 filed Jul. 27, 2018, which claims the benefit of U.S. provisional patent application No. 62/537,614, filed on Jul. 27, 2017, and entitled “A WAFER BONDING PIEZOELECTRIC FORCE SENSOR AND THE METHOD THEREOF,” the disclosures of which is expressly incorporated herein by reference in their entireties.

FIELD OF TECHNOLOGY

The present disclosure relates to microelectromechanical (“MEMS”) force sensing with piezoresistive and piezoelectric sensing elements integrated with complementary metal-oxide-semiconductor (“CMOS”) circuitry through a bonding process.

BACKGROUND

The combination of both piezoresistive and piezoelectric sensing elements for force sensing applications can provide both high sensitivity and offset information. The integration of the two sensing elements on a single die is challenging with existing foundry capabilities. This problem is exacerbated when various types of piezoelectric materials are included into the integration process. The foundry compatibility and thermal budgets of the process steps pose significant hurdles.

Accordingly, there is a desire to provide a solution that can address the integration issues stated above.

SUMMARY

The present disclosure pertains a MEMS force sensor including both piezoresistive and piezoelectric sensing elements integrated with digital circuitry on the same chip through bonding process. In one exemplary embodiment, both the piezoresistive and the piezoelectric elements are fabricated on one substrate first then bonded to another substrate which contains the CMOS integrated circuitry. In another exemplary embodiment, only the piezoelectric element is fabricated on one substrate while the piezoresistive element is fabricated on another substrate which contains the CMOS integrated circuitry. The piezoresistive element is realized through additional implant or diffusion steps in CMOS process.

An example microelectromechanical (“MEMS”) force sensor is described herein. The MEMS force sensor can include a sensor die configured to receive an applied force, where the sensor die can include a plurality of substrates bonded together. The MEMS force sensor can include a piezoresistive sensing element arranged on a first substrate of the sensor die, a piezoelectric sensing element arranged on the first substrate of the sensor die, and digital circuitry arranged on a second substrate of the sensor die. The piezoresistive sensing element can be configured to convert a strain to a first analog electrical signal that is proportional to the strain, and the piezoelectric sensing element can be configured to convert a change in strain to a second analog electrical signal that is proportional to the change in strain. Additionally, the digital circuitry can be configured to convert the first and second analog electrical signals to respective digital electrical output signals.

Another example microelectromechanical (“MEMS”) force sensor is described herein. The MEMS force sensor can include a sensor die configured to receive an applied force, where the sensor die can include a plurality of substrates bonded together. The MEMS force sensor can include a piezoresistive sensing element arranged on a first substrate of the sensor die, a piezoelectric sensing element arranged on a second substrate of the sensor die, and digital circuitry arranged on the first substrate of the sensor die. The piezoresistive sensing element can be configured to convert a strain to a first analog electrical signal that is proportional to the strain, and the piezoelectric sensing element can be configured to convert a change in strain to a second analog electrical signal that is proportional to the change in strain. Additionally, the digital circuitry can be configured to convert the first and second analog electrical signals to respective digital electrical output signals.

Additionally or additionally, in some implementations, the piezoresistive sensing element can at least partially overlaps with a solder ball. Alternatively or additionally, the piezoelectric sensing element can at least partially overlaps with a solder ball.

Alternatively or additionally, the piezoresistive sensing element can be formed by diffusion or implantation. In some implementations, the piezoresistive sensing element can be p-type formed on an n-type substrate. Alternatively, the piezoresistive sensing element can be n-type formed in a p-type well on an n-type substrate. In other implementations, the piezoresistive sensing element can be p-type formed in an n-type well on a p-type substrate. Alternatively, the piezoresistive sensing element can be n-type formed on a p-type substrate.

Alternatively or additionally, the piezoelectric sensing element can include at least one of aluminum nitride (AlN), zinc oxide (ZnO), lead zirconate titanate (PZT), lithium niobate (LiNbO3), barium titanate (BaTiO3), sodium potassium niobate (KNN), or polyvinylidene fluoride (PVDF).

An example method for manufacturing a microelectromechanical (“MEMS”) force sensor is described herein. The method can include forming a piezoresistive sensing element and a piezoelectric sensing element on a surface of a first substrate; forming digital circuitry on a surface of a second substrate; and bonding the first substrate and the second substrate to form a sensor die. The method can also include etching the first substrate and at least a portion of the second substrate to form a through hole; and creating an electrical connection between at least one of the piezoresistive sensing element or the piezoelectric sensing element formed on the first substrate and the digital circuitry formed on the second substrate via the through hole.

Additionally, the first substrate and the second substrate can be bonded using an adhesive layer.

Another example method for manufacturing a microelectromechanical (“MEMS”) force sensor is described herein. The method can include forming a piezoelectric sensing element on a surface of a first substrate; forming a piezoresistive sensing element and digital circuitry on a surface of a second substrate; and bonding the first substrate and the second substrate to form a sensor die. The method can also include etching the first substrate and at least a portion of the second substrate to form a through hole; and creating an electrical connection between the piezoelectric sensing element formed on the first substrate and the digital circuitry formed on the second substrate via the through hole.

Additionally, the first substrate and the second substrate can be bonded using a lower temperature fusion bonding process.

Alternatively or additionally, the step of creating an electrical connection can further include depositing an isolation/barrier layer on at least a portion of a surface of the sensor die; and depositing a metal layer on at least a portion of a surface of the isolation/barrier layer.

Alternatively or additionally, the method can further include depositing a polymer layer on the sensor die, wherein the polymer layer covers the isolation/barrier layer and the metal layer; and etching the polymer layer to form an under bump metallization (UBM) layer.

Alternatively or additionally, the method can further include depositing a second metal layer on the UBM layer.

Alternatively or additionally, the method can further include depositing a solder ball on the second metal layer.

Alternatively or additionally, the first substrate can include a carrier substrate. The method can further include removing the carrier substrate after bonding the first substrate and the second substrate.

Other systems, methods, features and/or advantages will be or may become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features and/or advantages be included within this description and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE FIGURES

The components in the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding parts throughout the several views. These and other features of will become more apparent in the detailed description in which reference is made to the appended drawings wherein:

FIG. 1 illustrates a MEMS substrate with piezoresistive and piezoelectric sensing elements and a carrier substrate according to an implementation described herein.

FIG. 2 illustrates bonding of the MEMS and carrier substrates shown in FIG. 1 to a CMOS substrate with integrated circuitry according to an implementation described herein.

FIG. 3 illustrates removal of the carrier substrate to create an intermediate MEMS substrate and creation of an isolation/barrier layer and metal plating for electrical interconnection according to an implementation described herein.

FIG. 4 illustrates a cross-sectional view of the completed MEMS force sensor including the MEMS substrate with the piezoresistive and piezoelectric sensing elements bonded to the CMOS substrate with integrated circuitry according to an implementation described herein.

FIG. 5 illustrates a MEMS substrate with a piezoelectric sensing element and a CMOS substrate with integrated circuitry and a piezoresistive sensing element according to an implementation described herein.

FIG. 6 illustrates bonding of the MEMS substrate and the CMOS substrate shown in FIG. 5, removal of the carrier substrate and dielectric layer, and creation of metallization for the piezoelectric sensing element according to an implementation described herein.

FIG. 7 illustrates creation of an isolation/barrier layer and metal plating for electrical interconnection according to an implementation described herein.

FIG. 8 illustrates a cross-sectional view of the completed MEMS force sensor including the MEMS substrate with the piezoelectric sensing element bonded to the CMOS substrate with integrated circuitry and the piezoresistive sensing element according to an implementation described herein.

DETAILED DESCRIPTION

The present disclosure can be understood more readily by reference to the following detailed description, examples, drawings, and their previous and following description. However, before the present devices, systems, and/or methods are disclosed and described, it is to be understood that this disclosure is not limited to the specific devices, systems, and/or methods disclosed unless otherwise specified, and, as such, can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting.

The following description is provided as an enabling teaching. To this end, those skilled in the relevant art will recognize and appreciate that many changes can be made, while still obtaining beneficial results. It will also be apparent that some of the desired benefits can be obtained by selecting some of the features without utilizing other features. Accordingly, those who work in the art will recognize that many modifications and adaptations may be possible and can even be desirable in certain circumstances, and are contemplated by this disclosure. Thus, the following description is provided as illustrative of the principles and not in limitation thereof.

As used throughout, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a sensing element” can include two or more such sensing elements unless the context indicates otherwise.

The term “comprising” and variations thereof as used herein is used synonymously with the term “including” and variations thereof and are open, non-limiting terms.

Ranges can be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another aspect includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another aspect. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

As used herein, the terms “optional” or “optionally” mean that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not.

Referring to FIGS. 1-4, the process of bonding a MEMS substrate and a CMOS substrate to create an integrated MEMS force sensor is shown. According to one implementation, the process starts with a MEMS substrate 102 including both piezoresistive and piezoelectric sensing elements and a carrier substrate 101 as shown in FIG. 1. The carrier substrate 101 includes a handle substrate 103 and a MEMS adhesive layer 104. The handle substrate 103 can be made of silicon or other semiconductor material (e.g., gallium arsenide (GaAs)) or glass. The MEMS adhesive layer 104 can be SU-8, PMMA, or other polymer like material. The MEMS substrate 102 includes a sensor substrate 105, a planarized dielectric layer 106, a passivation layer 107, a piezoresistive sensing element 112, and a piezoelectric sensing element. In other words, the piezoelectric and piezoresistive sensing elements are integrated on the same die (i.e., the MEMS substrate 102). The piezoelectric sensing element includes a piezoelectric layer 109 arranged between opposing electrodes 108, 110 (e.g., bottom electrode 108 and top electrode 110). Additionally, the piezoresistive sensing element 112, which is a lightly doped region, is arranged between at least two heavily doped regions 111. A metal interconnection and contact 113 is used to provide electrical connection to the piezoresistive sensing element 112. Although one piezoelectric sensing element and one piezoresistive sensing element are shown in FIG. 1 as examples, this disclosure contemplates that the MEMS substrate 102 can include more than one piezoelectric sensing elements and/or more than one piezoresistive sensing element.

In FIGS. 1-4, the sensor substrate 105 can be made of an n-type semiconductor material such as n-type silicon or other semiconductor (e.g., gallium arsenide (GaAs) or silicon carbide (SiC)). In these implementations, the piezoresistive sensing element 112 can be formed by diffusion or implantation onto the n-type substrate. For example, as shown in FIG. 1, the piezoresistive sensing element 112 can be a lightly doped p-type region arranged between heavily doped p-type regions 111. Alternatively, the piezoresistive sensing element 112 can be a lightly doped n-type region arranged between heavily doped n-type regions 111, all formed inside a p-type well (not shown). In other implementations, the sensor substrate 105 can be made of a p-type semiconductor material such as p-type silicon or other semiconductor (e.g., gallium arsenide (GaAs) or silicon carbide (SiC)). In these implementations, the piezoresistive sensing element 112 can be formed by diffusion or implantation onto the p-type substrate. For example, as shown in FIG. 1, the piezoresistive element 112 can be a lightly doped n-type region arranged between heavily doped n-type regions 111. Alternatively, the piezoresistive sensing element 112 can be a lightly-doped p-type region arranged between heavily doped p-type regions 111, all formed in a n-type well (not shown). Optionally, the piezoresistive sensing element 112 can be fully or partially covered by a solder ball (or solder bump), which is placed as described below with regard to FIG. 4. In other words, the respective areas of the sensor die on which the piezoresistive sensing element and solder ball are located can fully or partially overlap with one another. This is where the maximum response is available.

The piezoresistive sensing element 112 can change resistance in response to deflection of a portion of the MEMS substrate 102. For example, as strain is induced in the MEMS substrate 102 proportional to the force applied to a MEMS force sensor, a localized strain is produced on a piezoresistive sensing element such that the piezoresistive sensing element experiences compression or tension, depending on its specific orientation. As the piezoresistive sensing element compresses and tenses, its resistivity changes in opposite fashion. Accordingly, a Wheatstone bridge circuit including a plurality (e.g., four) piezoresistive sensing elements (e.g., two of each orientation relative to strain) becomes unbalanced and produces a differential voltage. This differential voltage is directly proportional to the force applied to the MEMS force sensor. This differential voltage can represent the static force applied to the MEMS force sensor. This disclosure contemplates that this differential voltage can be received at and processed by digital circuitry (described herein). For example, the digital circuitry can be configured to, among other functions, convert an analog electrical signal to a digital electrical output signal. Example MEMS force sensors using piezoresistive sensing elements are described in U.S. Pat. No. 9,487,388, issued Nov. 8, 2016 and entitled “Ruggedized MEMS Force Die;” U.S. Pat. No. 9,493,342, issued Nov. 15, 2016 and entitled “Wafer Level MEMS Force Dies;” U.S. Pat. No. 9,902,611, issued Feb. 27, 2018 and entitled “Miniaturized and ruggedized wafer level mems force sensors;” and U.S. Patent Application Publication No. 2016/0363490 to Campbell et al., filed Jun. 10, 2016 and entitled “Ruggedized wafer level mems force sensor with a tolerance trench,” the disclosures of which are incorporated by reference in their entireties.

As described above, the piezoelectric sensing element can include the piezoelectric layer 109 arranged between opposing electrodes 108, 110 (e.g., bottom electrode 108 and top electrode 110). The piezoelectric layer 109 can be made of at least one of aluminum nitride (AlN), zinc oxide (ZnO), lead zirconate titanate (PZT), lithium niobate (LiNbO₃), barium titanate (BaTiO₃), sodium potassium niobate (KNN), or polyvinylidene fluoride (PVDF). When a force is applied to a MEMS force sensor, the strain is transferred to piezoelectric element 109, which converts the strain into charge. In other words, the piezoelectric element 109 can change an electrical characteristic (i.e., charge) in response to deflection of a portion of the MEMS substrate 102. Thus, the change in electrical characteristic can be detected as an analog electrical signal (e.g., change in voltage) at the opposing electrodes 108, 110. This disclosure contemplates that this analog electrical signal can be received at and processed by digital circuitry (described herein). For example, the digital circuitry can be configured to, among other functions, convert an analog electrical signal to a digital signal. The voltage measured at the opposing electrodes 108, 110 of the piezoelectric sensing element can represent the dynamic force applied to the MEMS force sensor. This information can be used in conjunction with the differential voltage signal described above, which represents the static force applied to the MEMS force sensor, to improve sensitivity and accuracy of the MEMS force sensor. An example integrated fusion force sensor is described in detail in PCT/US2018/017572, filed on Feb. 9, 2018, and entitled “INTEGRATED PIEZORESISTIVE AND PIEZOELECTRIC FUSION FORCE SENSOR,” the disclosure of which is incorporated by reference in its entirety. Optionally, the piezoelectric sensing element can be fully or partially covered by a solder ball, which is placed as described below with regard to FIG. 4. In other words, the respective areas of the sensor die on which the piezoelectric sensing element and solder ball are located can fully or partially overlap with one another. This is where the maximum response is available.

The MEMS substrate 102 and the carrier substrate 101 can be bonded together. For example, the MEMS substrate 102 and the carrier substrate 101 can be bonded together using the MEMS adhesive layer 104. The bonded MEMS and carrier substrates forms an intermediate MEMS substrate 214 as shown in FIG. 2. Then, the intermediate MEMS substrate 214 can be bonded to a CMOS substrate 215. The bonded MEMS and CMOS substrates form a sensor die. The CMOS substrate 215 includes an adhesive layer 216, a circuit substrate 217, a planarized dielectric layer 219, and a metal interconnection and contact 218. The adhesive layer 216 can be made of SU-8, PMMA, or other polymer like material. As shown in FIG. 2, the intermediate MEMS substrate 214 can be bonded to the CMOS substrate 215 using the adhesive layer 216. This disclosure contemplates that a plurality of an n-type metal-oxide-semiconductor (nMOS) transistors and/or a p-type metal-oxide-semiconductor (pMOS) transistors can be fabricated on the circuit substrate 217 using CMOS processes, which are known in the art. The circuit substrate 217 can be made of silicon. Although a silicon substrate is provided as an example, this disclosure contemplates that the circuit substrate 217 can be made from a material other than silicon (e.g., glass, ceramic, etc.). The nMOS and pMOS transistors can form digital circuitry (also sometimes referred to as “integrated circuits (IC)”). The nMOS and pMOS transistors can form various components of the digital circuitry. The digital circuitry can optionally include other components including, but not limited to, bipolar transistors; metal-insulator-metal (“MIM”) and metal-oxide-semiconductor (“MOS”) capacitors; diffused, implanted, and polysilicon resistors; and/or diodes. The digital circuitry can include, but is not limited to, one or more of a differential amplifier or buffer, an analog-to-digital converter, a clock generator, non-volatile memory, and a communication bus. The metal interconnection and contact 218 can be used to provide electrical connection to the digital circuitry. It should be understood that the number of metal interconnection and contacts shown in FIG. 2 (i.e., two) are provided only as an example.

As shown in FIG. 3, the carrier substrate and the MEMS adhesive layer can be removed, which leaves a bonded MEMS substrate 320. Thereafter, a through hole 324 can be created by etching through the bonded MEMS substrate 320 and the adhesive layer 216 of the CMOS substrate 215 until the metal interconnect and contact 218 is exposed. Although a single through hole 324 is shown in FIG. 3, it should be understood that the MEMS force sensor can include more than one through hole. The through hole(s) facilitate providing an electrical connection between the force sensing elements (e.g., the piezoresistive and/or piezoelectric sensing elements), which are fabricated on the bonded MEMS substrate 320, and the digital circuitry, which is arranged on the CMOS substrate 215. After creating the through hole 324, an isolation/barrier layer 321 can be deposited onto portions of the bonded MEMS substrate 320, followed by deposition of a metal layer 322 (e.g., copper) onto portions of the isolation/barrier layer 321. The metal layer 322 can be deposited in desired locations using a plating mask layer 323 as shown in FIG. 3.

A cross section of the completed MEMS force sensor is shown in FIG. 4. As shown in this figure, the plating mask layer can be removed, and a polymer layer 425 can be deposited onto the bonded MEMS substrate 320 and then etched to create an under bump metallization (UBM) layer 426. A metal layer 427 (e.g., copper) can be disposed on the UBM layer 426. Then, a solder ball 428 can be placed as depicted in FIG. 4. It should be understood that the solder ball 428 can be used to provide electrical and/or mechanical connection between the MEMS force sensor depicted in FIG. 4 and an external device (e.g., an electronic device) or external substrate.

Referring now to FIGS. 5-8, another process of bonding a MEMS substrate and a CMOS substrate to create an integrated MEMS force sensor is shown. According to this implementation, the process starts with a MEMS substrate 502 including a piezoelectric sensing element and a carrier substrate 515 as shown in FIG. 5. The MEMS substrate 502 further includes a first planarized dielectric layer 514, a piezoelectric layer 513, a bottom electrode 512, and a second planarized dielectric layer 511. The piezoelectric layer 513 can be made of at least one of aluminum nitride (AlN), zinc oxide (ZnO), lead zirconate titanate (PZT), lithium niobate (LiNbO₃), barium titanate (BaTiO₃), sodium potassium niobate (KNN), or polyvinylidene fluoride (PVDF).

As shown in FIG. 5, the MEMS substrate 502 can be bonded to a CMOS substrate 501 using a lower temperature fusion bonding process such that performance of the circuitry integrated in the CMOS substrate 501 (e.g., digital circuitry described herein) is not affected during the bonding process. The bonded MEMS and CMOS substrates form a sensor die. The CMOS substrate 501 can include a circuit substrate 503, a piezoresistive sensing element 505, an intermetal dielectric (IMD) layer 507, a metal layer 508, a passivation layer 509, a planarized dielectric layer 510, and a via 506. The via 506 can connect the metal layer 508 to the digital circuitry formed on the circuit substrate 503. It should be understood that the numbers of vias and/or metal layers shown in FIGS. 5-8 are provided only as examples and that this disclosure contemplates that more or less vias and/or metal layers can be used to provide electrical connection to the digital circuitry.

This disclosure contemplates that a plurality of an n-type metal-oxide-semiconductor (nMOS) transistors and/or a p-type metal-oxide-semiconductor (pMOS) transistors can be fabricated on the circuit substrate 503 using CMOS processes, which are known in the art. The circuit substrate 503 can be made of silicon. Although a silicon substrate is provided as an example, this disclosure contemplates that the circuit substrate 503 can be made from a material other than silicon (e.g., glass, ceramic, etc.). The nMOS and pMOS transistors can form digital circuitry. The nMOS and pMOS transistors can form various components of the digital circuitry. The digital circuitry can optionally include other components including, but not limited to, bipolar transistors; metal-insulator-metal (“MIM”) and metal-oxide-semiconductor (“MOS”) capacitors; diffused, implanted, and polysilicon resistors; and/or diodes. The digital circuitry can include, but is not limited to, one or more of a differential amplifier or buffer, an analog-to-digital converter, a clock generator, non-volatile memory, and a communication bus.

The piezoresistive sensing element 505, which is a lightly doped region, is arranged between at least two heavily doped regions 504. A metal interconnection and contact can be used to provide electrical connection to the piezoresistive element 505. Although one piezoresistive sensing element is shown in the figures as an example, this disclosure contemplates that the CMOS substrate 501 can include more than one more than one piezoresistive sensing element. The piezoresistive sensing element 505 and heavily doped regions 504 can be formed on the circuit substrate 503 by deposition or implantation process on p-type or n-type substrates as described above. Additionally, the piezoresistive sensing element 505 can be formed using processes compatible with the CMOS processes used to form the digital circuitry. Formation of piezoresistive sensing elements with processes compatible with CMOS processes are described in detail in PCT/US2018/017564, filed on Feb. 9, 2018, and entitled “INTEGRATED DIGITAL FORCE SENSORS AND RELATED METHODS OF MANUFACTURE.” Further, piezoresistive force sensing is described in detail above with regard to FIG. 14. Optionally, the piezoresistive sensing element 505 can be fully or partially covered by a solder ball (or solder bump), which is placed as described below with regard to FIG. 8. In other words, the respective areas of the sensor die on which the piezoresistive sensing element and solder ball are located can fully or partially overlap with one another. This is where the maximum response is available.

Referring now to FIG. 6, the carrier substrate and the first planarized dielectric layer can be removed. Thereafter, a top electrode 619, another (e.g., third) planarized dielectric layer 617, a passivation layer 618, and a metal interconnect and contact 620 can be formed on the MEMS substrate. This results in a bonded MEMS substrate 616 as shown in FIG. 6. As depicted in the figure, a piezoelectric sensing element can include the piezoelectric layer 513 arranged between opposing electrodes 512, 619 (e.g., bottom electrode 512 and top electrode 619). Although one piezoelectric sensing element is shown in the figures as an example, this disclosure contemplates that the bonded MEMS substrate 616 can include more than one piezoelectric sensing element. Piezoelectric force sensing is described in detail above with regard to FIG. 14. Optionally, the piezoelectric sensing element can be fully or partially covered by a solder ball, which is placed as described below with regard to FIG. 8. In other words, the respective areas of the sensor die on which the piezoelectric sensing element and solder ball are located can fully or partially overlap with one another. This is where the maximum response is available.

Referring now to FIG. 7, a through hole 724 can be created by etching through the bonded MEMS substrate 616 and portions of the CMOS substrate 501 (e.g., the planarized dielectric layer 510) until the metal layer 508 is exposed. Although a single through hole 724 is shown in FIG. 7, it should be understood that the MEMS force sensor can include more than one through hole. The through hole(s) facilitate providing an electrical connection between the force sensing elements (e.g., the piezoelectric sensing elements), which are fabricated on the bonded MEMS substrate 616, and the piezoresistive sensing element 505 and/or digital circuitry, which are arranged on the CMOS substrate 501. After creating the through hole 724, an isolation/barrier layer 723 can be deposited onto portions of the bonded MEMS substrate 616, followed by deposition of a metal layer 722 (e.g., copper) onto portions of the isolation/barrier layer 723. The metal layer 722 can be deposited in desired locations using a plating mask layer 721 as shown in FIG. 7.

A cross section of the completed MEMS force sensor is shown in FIG. 8. As shown in this figure, the plating mask layer can be removed, and a polymer layer 725 can be deposited onto the bonded MEMS substrate 616 and then etched to create an under bump metallization (UBM) layer 826. A metal layer 827 (e.g., copper) can be disposed on the UBM layer 826. Then, a solder ball 828 can be placed as depicted in FIG. 8. It should be understood that the solder ball 828 can be used to provide electrical and/or mechanical connection between the MEMS force sensor depicted in FIG. 8 and an external device (e.g., an electronic device) or external substrate.

Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims. 

The invention claimed is:
 1. A microelectromechanical (“MEMS”) force sensor, comprising: a sensor die configured to receive an applied force, wherein the sensor die comprises a plurality of substrates bonded together, a piezoresistive sensing element arranged on a first substrate of the sensor die, wherein the piezoresistive sensing element is configured to convert a strain to a first analog electrical signal that is proportional to the strain, a piezoelectric sensing element arranged on the first substrate of the sensor die, wherein the piezoelectric sensing element is configured to convert a change in strain to a second analog electrical signal that is proportional to the change in strain, and digital circuitry arranged on a second substrate of the sensor die, wherein the digital circuitry is configured to convert the first and second analog electrical signals to respective digital electrical output signals.
 2. The MEMS force sensor of claim 1, wherein the piezoresistive sensing element at least partially overlaps with a solder ball.
 3. The MEMS force sensor of claim 1, wherein the piezoelectric sensing element at least partially overlaps with a solder ball.
 4. The MEMS force sensor of claim 1, wherein the piezoresistive sensing element is formed by diffusion or implantation.
 5. The MEMS force sensor of claim 1, wherein the piezoresistive sensing element is p-type formed on an n-type substrate.
 6. The MEMS force sensor of claim 1, wherein the piezoresistive sensing element is p-type formed in an n-type well on a p-type substrate.
 7. The MEMS force sensor of claim 1, wherein the piezoresistive sensing element is n-type formed on a p-type substrate.
 8. The MEMS force sensor of claim 1, wherein the piezoresistive sensing element is n-type formed in a p-type well on an n-type substrate.
 9. The MEMS force sensor of claim 1, wherein the piezoelectric sensing element comprises at least one of aluminum nitride (AlN), zinc oxide (ZnO), lead zirconate titanate (PZT), lithium niobate (LiNbO3), barium titanate (BaTiO3), sodium potassium niobate (KNN), or polyvinylidene fluoride (PVDF).
 10. A microelectromechanical (“MEMS”) force sensor, comprising: a sensor die configured to receive an applied force, wherein the sensor die comprises a plurality of substrates bonded together, a piezoresistive sensing element arranged on a first substrate of the sensor die, wherein the piezoresistive sensing element is configured to convert a strain to a first analog electrical signal that is proportional to the strain, a piezoelectric sensing element arranged on a second substrate of the sensor die, wherein the piezoelectric sensing element is configured to convert a change in strain to a second analog electrical signal that is proportional to the change in strain, and digital circuitry arranged on the first substrate of the sensor die, wherein the digital circuitry is configured to convert the first and second analog electrical signals to respective digital electrical output signals.
 11. The MEMS force sensor of claim 10, wherein the piezoresistive sensing element at least partially overlaps with a solder ball.
 12. The MEMS force sensor of claim 10, wherein the piezoelectric sensing element at least partially overlaps with a solder ball.
 13. The MEMS force sensor of claim 10, wherein the piezoresistive sensing element is formed by diffusion or implantation.
 14. The MEMS force sensor of claim 10, wherein the piezoresistive sensing element is p-type formed on an n-type substrate.
 15. The MEMS force sensor of claim 10, wherein the piezoresistive sensing element is p-type formed in an n-type well on a p-type substrate.
 16. The MEMS force sensor of claim 10, wherein the piezoresistive sensing element is n-type formed on a p-type substrate.
 17. The MEMS force sensor of claim 10, wherein the piezoresistive sensing element is n-type formed in a p-type well on an n-type substrate.
 18. The MEMS force sensor of claim 10, wherein the piezoelectric sensing element comprises at least one of aluminum nitride (AlN), zinc oxide (ZnO), lead zirconate titanate (PZT), lithium niobate (LiNbO3), barium titanate (BaTiO3), or sodium potassium niobate (KNN). 